1. Field of the Invention
The invention generally relates to the data communication for Selective Acknowledgement (SACK) operation, and more particularly, to design the header in the Medium Access Control (MAC) layer for transmitting data.
2. Description of the Related Art
In the current SACK scheme, the base station may send back a SACK Order over F-PCH to indicate the mobile station which frames should be retransmitted. Thus, it's very important for the base station side to know the total number of Reverse-Enhanced Access Channel (R-EACH) data frames in the message and the mobile station address information, so that the base station can set the SELECTIVE_ACK_BIT_MASK properly and the SACK Order can be delivered over a paging channel.
But according to Link Access control (LAC) layer specifications, the current SACK scheme may meet some problems. At the mobile station side, when sending a message, the Segmentation and Reassembly (SAR) sub-layer adds a header to indicate the length of the encapsulated Protocol Data Unit (PDU), and the header will be sent in the first frame. The parameter MSG_LENGTH indicates the length of the encapsulated PDU. However, at the base station side, the LAC layer doesn't know how many R-EACH data frames are sent in the probe even if SAR sub-layer receives the first frame because the R-EACH data rate information is not delivered to the SAR sub-layer through the primitive MAC-Data Indication, such as channel_id, channel_type, data, size, system_time, physical_channel_id.
In addition, in the mobile station, when sending a message, the Addressing sub-layer adds the information of the mobile station identification. Since the SACK Order is delivered over the paging channel, the mobile station's address information is very important. However currently, in the base station side, the SAR sub-layer doesn't allow the PDU frame to be passed to upper layers. Only if the encapsulated PDU passes the Cyclic Redundancy Check (CRC), the PDU is delivered to upper layers. This means that according to the current spec, only after the SAR sub-layer gets a valid PDU, the mobile station's address information can be obtained. But if a valid PDU is received, it means all data frames are received correctly and there is no need to send the SACK Order.
In addition, lower data rates are introduced for R-EACH in the Rev F standard. This can make it more difficulties to get the message length and mobile address information. According to Table 2.1.3.5.2-1 of C.S0002, in Rev F, the minimum number of information bits in one R-EACH data frame is just 16 bits, which corresponds to the rate of 1.5 kbps This means that the first data frame (or frame) might be not enough to carry the message length and mobile station address information. For a rate of 1.5 kbps rate, one data frame is not even enough for the message length information. According to Section 2.1.1.5 in C.S0004, for R-EACH, the message length information plus SI bits will be 2(SI)+1(EXT_MSG_LENGTH)+7 or 15(MSG_LENGTH), which is 10 or 18 bits. Thus it is difficult for the base station side to know how many frames are enough to get the message length and address information.